A method and an arrangement of this kind for checking the operability of a tank-venting system is disclosed, for example, in U.S. Pat. No. 5,890,474 as well as in U.S. Pat. No. 6,131,550.
From U.S. Pat. No. 6,131,550, a method is known wherein the time-dependent trace of at least one operating characteristic variable is detected during standstill of the vehicle and this variable is compared to a previously measured, computed or estimated time-dependent trace of the at least one operating characteristic variable which is not disturbed by a tanking operation. A fault announcement is not outputted when the detected time-dependent trace deviates by a pregivable value from the diagnostic trace. The operating characteristic variable is advantageously the electric current of an overpressure pump with which an overpressure is generated in the tank-venting system.
For this method, it is problematic that the time-dependent trace of the operating characteristic variable must be continuously detected and compared to a computed, stored or measured operating characteristic variable. A leak is here only detected when a significant change of the operating characteristic value is detected within this diagnostic trace.
It is not infrequent that the driver of a motor vehicle, for example, stops at a gas station, switches off the engine, opens the cap of the tank but does not immediately start tanking. In this case, the tightness check starts with the standstill of the vehicle and erroneously determines a leak because of the removed tank cap which leak is no longer present after the tanking operation and the closing of the tank. In a case such as this, an erroneous entry into a memory of a control apparatus or the like should be avoided. In the arrangement disclosed in U.S. Pat. No. 6,131,550, an opening of the tank can only be detected when this takes place during the course of a diagnosis. For this reason, a tank, which has already been opened in advance of the diagnostic sequence, cannot be distinguished from an actually present leak in the tank-venting system.